Immunosequestration: A new technique for studying peripheral iodothyronine metabolism in vitro

Studies of the pathways of peripheral metabolism of iodothyronines in vitro are often obscured quantitatively and qualitatively, both by the rapid degradation of products of interest and by the generation of a single product from more than one precursor. To overcome these problems, we tested the ability of rabbit immunoglobulin G containing antibodies against particular iodothyronines to inhibit the enzymatic degradation of their respective antigens. Such inhibition was indeed demonstrable in the case of all the iodothyronines tested, i.e. T₃, rT₃, and 3, 3'-diiodothyronine. The general method was then applied to studies of the ontogeny of peripheral iodothyronine metabolism in homogenates of chick embryo liver. By appropriate selection of antibodies and ¹²⁵I-labeled substrates, it was shown that livers from immature embryos (<19–20 days of age) deiodinate T₄ rapidly and almost entirely to rT₃, with very little T₃ being formed. In livers from embryos that had matured spontaneously or had been injected with hydrocortisone, T₄ was deiodinated more slowly, but almost exclusively to T_3. The findings indicate that the maturation of hepatic T₄ metabolism involves an increase in the rate of 5′-monodeiodination coupled with a proportionately greater decrease in 5-monodeiodination, so that formation of T₃ from T₄ is increased and formation of rT₃ is greatly retarded. The new technique, which we have termed immunosequestration, thus provided proof of conclusions reached only by inference in earlier studies that employed the in vitro techniques used in the past. The technique should prove valuable in studies of other problems related to peripheral iodothyronine metabolism and in other metabolic studies as well.